1. Time-resolved and steady-state measurements of fuorescence anisotropy show that certain dye labels on proteins have a viscosity independent rotational freedom which is temperature dependent. A major conclusion is that such dyes probe the surface undulations of a protein. Fluorescent groups in the interior show little independent rotational freedom. 2. The interaction of serum albumins with aryl esters results in the acylation of the proteins. The reaction was studied with fluorescent and non-fluorescent substrates. The rate of reaction as shown by stopped-flow kinetics is faster for human than bovine serum albumin. The rate with acetylsalicylic acid (asprin) is slow and may require a catalyst. Structure-activity studies have been carried out with various substrates and inhibitors. 3. Energy transfer has been used to estimate the distance between the sole tryptophan and the single sulfhydryl group of human serum albumin. Donor-acceptor orientations have been studied by polarization methods.